The invention concerns a process for the catalytic hydration of lower olefins, e.g. especially ethylene or propylene, and also butenes, on a solid bed catalyst to form the corresponding alcohols in the gas phase or gas-liquid phase.
It is in the prior art to make alcohols by the direct gas-phase hydration of olefins. The customary technique consists in operating not only at elevated pressure but also with very great amounts of circulating gas owing to the unfavorable equilibrium situation at the reaction temperature that are applied.
In the prior art process it is common practice to keep the concentration of the circulating gas at approximately 85% by volume. This is based on the consideration that, with a higher circulating gas concentration, the transformation at the catalyst increases while the energy cost and invested costs are lower, but at the same time the amount of gas that has to be purged increases considerably because the impurities can accumulate in the circulating gas to only a slight degree. If, however, a lower concentration of olefin in the circulating gas is used, i.e., a substantially higher concentration of impurities in the circulating gas, the amount of gas that has to be purged decreases considerably. The use of circulating gas concentrations substantially below 85% by volume is not considered, however, because in that case the olefin transformation per passage, under otherwise constant conditions, diminishes virtually in proportion to the reduction of the olefin concentration in the circulating gas.
The impurities that have to be purged from the circulating gas are, on the one hand, substances which are brought in with the olefin input, and, on the other hand, by-products which form during the hydrations of olefins. The impurities brought in with the olefin input are substances of which some boil lower than the olefin--methane and nitrogen in the case of ethylene (ethanol production)--and some boil higher, such as ethane and C.sub.3 hydrocarbons; in the case of propylene (isopropanol production) the impurities include ethylene in addition to methane and nitrogen as the substances which boil lower than propylene, and the higher boiling impurities are propane and C.sub.4 hydrocarbons. The by-products that form during the hydration are particularly olefin polymers of low molecular weight, such as dimers, trimers and tetramers, which can form the corresponding alcohols during the reaction at the catalyst, and the corresponding saturated hydrocarbons.
Separation of these alcohols from the principal alcohol product is not easy. The olefin polymers of low molecular weight, however, can also be further polymerized to higher hydrocarbons which are entrained in the form of fine mists with the circulation gas and are extremely difficult to separate. By coating the catalyst, they reduce its activity and shorten its life.
The gas that is purged has hitherto been burned off, or it has been used in a system permitting the utilization of olefins of low concentration (e.g., alkylation of benzene), or it has been fed back into the olefin producing system, or it has been put through a distillation process at the plant.
The distillation process, according to the state of the art, is performed at low to medium pressures and correspondingly low temperatures, i.e., under conditions that are far below the critical characteristics of the olefins. The disadvantage of this procedure is to be seen in the fact, among others, that multi-stage refrigeration units have to be used for the condensation, along with columns of relatively large radius; also, on account of the higher heat of evaporation of the olefins, a high expenditure of energy is required at low temperatures for the evaporation and condensation. In the distillative separating process that has hitherto been customary in the art (Ullmann, "Enzyklopadieder technischem Chemie," 3rd Edition, Vol. 10, pp. 150-161) for low-boiling olefins, two columns are necessary for the separation of low boiling and high boiling impurities, the low boiling impurities being separated in the first column, while in the second column the higher boiling impurities are drawn off from the sump of the column and the concentrated olefin is drawn off at the head of the column.